1. Field of the Invention:
The present invention relates generally to a cellular mobile communication system, and in particular, to a method and apparatus for generating a handover neighbor list of base stations capable of communicating with a mobile terminal.
2. Description of the Related Art:
In general, a cellular mobile communication system divides its entire service area into a plurality of base station (BS) areas to form cells which are service areas of relatively smaller size than the entire service area, and controls such base stations on a centralized basis with a switching system, so that a subscriber can continue a call seamlessly using a mobile terminal even while moving between cells. When a mobile terminal travels from a current cell of a particular base station to a new cell in the cellular system, a handover occurs to continue a call. Establishment of a new channel due to the handover is referred to as an “ADD,” while release of an existing channel due to the handover is referred to as a “DROP.”
Handovers can exhibit different efficiencies in terms of reliability of call continuity and system load according to the method used to perform the handover, and implementation contents. A handover method can be categorized into two types: a soft handover and a hard handover. The soft handover is a make-before-cut method, which is a typical handover scheme for a code division multiple access (CDMA) system, in which a new call is made before a current call is cut, while the hard handover is a cut-before-make method in which a current call is cut before a new call is made. When a handover is required, the CDMA system first performs the soft handover. However, in some cases, the CDMA system guarantees continuity of a call through hard handover.
In the CDMA system, base stations are identified by pilot signals transmitted over a code channel. A pilot signal transmitted over a pilot channel is spread by a pseudo-random noise (PN) code, known as a non-modulated direct-sequence signal, which is continuously broadcast by each CDMA base station. Each base station uses a unique time offset of a PN code (hereinafter referred to as a “PN offset”). A mobile terminal acquires timing of a forward CDMA channel by the pilot signal, measures signal strengths between base stations and compares the signal strengths in order to determine the time when a handover is required. Searching pilot signals from all the base stations to determine handover is very inefficient, causing a waste of time. Therefore, a mobile terminal manages a neighbor list specifying pilot signals (specifically, PN offsets) from base stations which are expected to be able to communicate with the mobile terminal.
Handover is a procedure performed as a mobile terminal moves from one cell to another cell. Therefore, a base station controller (BSC) approves handover only to the base stations having pilot signals in the neighbor list, for a mobile terminal requesting the handover. A typical neighbor list is ranked in view of quality of pilot signals, e.g., carrier-to-interference ratio (C/I). In another case, the typical neighbor list is manually made by a system operator.
Managing these neighbor lists is undesirable, however, because they cannot reflect various CDMA characteristics, such as regional characteristic, receiver type and capability of simultaneously demodulating multiple signals. Therefore, there is a demand for a method of determining a neighbor list of base stations which are expected to be able to best communicate with a mobile terminal, in consideration of the form in which handover of the mobile terminal actually occurs.